Although adding silica to rubber compositions as a reinforcing filler is known, generally, silica has a tendency to aggregate due to the formation of hydrogen bonds with silanol groups present on the surface of the particles thereof. Accordingly, there is a disadvantage in that kneading time normally needs to be long in order to satisfactorily disperse the silica in the rubber component. Also, when the dispersion of silica in the rubber component is insufficient, there is a disadvantage in that the Mooney viscosity of the obtained rubber composition increases, resulting in insufficient workability. Furthermore, since basic substances normally used as vulcanization accelerators are adsorbed by the surface of the silica particles due to their being acidic, there is a disadvantage in that control of the vulcanization process of the rubber composition and control of physical properties, especially viscoelastic characteristics, of the obtained vulcanized rubber becomes difficult. With respect to improving vulcanization characteristics, for example, Japanese Patent Publication (A) No. 2003-138077 suggests increasing cross-linking efficiency by adding a specific carboxylic acid amine salt as a vulcanization accelerator to a halogen-based saturated rubber. Also, with respect to improving workability and viscoelastic characteristics after vulcanization, for example, Japanese Patent Publication (A) No. 2003-176378 suggests adding, as a vulcanization accelerator, a compound having one or more each of an amine group and a group derived from an unsaturated carboxylic acid to natural rubber and/or a diene based synthetic rubber. However, these conventional techniques cannot improve all of the silica dispersibility (that is, workability of the rubber composition), vulcanization characteristics of the rubber composition, and viscoelastic characteristics of the rubber composition after vulcanization in good balance in a rubber composition including natural rubber and/or a diene-based synthetic rubber. Accordingly, high-dimensional and well-balanced improvement of these characteristics is desired.